HDD-Task to Let “Green” Power Flow Under a River

From 2009 through 2012, work will be carried out on the Rhine power plant Iffezheim (RKI) to be extended with a fifth turbine. Directly on the Rhine crossing of the B 500 to France, the largest hydropower plant in Germany and one of the largest in Europe is emerging. The RKI, a joint partnership of the EnBW, as well as the EDF and operators of the hydropower plant, will have invested in around 90 million Euro and produce annually further 122 million kWh C0²-free, environmental friendly power.

The company WMZ from Wittislingen plays a major part in the power flowing from the turbine to the sub-station on the other side of the Rhine. The bore company had an imaginative concept and therefore received a contract to install five multi-pipe bundles, each over a length of 220 m beneath the Rhine.

Not a Routine Task

This bore task is not a routine task for several reasons. Many of the companies that were asked about this project were of the impression, that a bore with sufficient quality was not possible within the bore area, directly within the undercurrent of the sluice. It is well known that the undercurrent beneath the river bed is difficult to bore. Sand, pebbles and rubble in all grain sizes are apparent and have to be handled safely. There were high demands on the drilling fluid, due to the penetrable structure. There were numerous metallic objects underground, which originated from the set up of the barrage, sluice and power plant at Iffezheim from the 1970s, which meant that standard detection systems could not be applied due to the busy water traffic and the interferences, caused by water terminals.
 
The bore company WMZ from Wittislingen, with their imaginative concept and the right bore technology managed to convince the contractor, Arge Implenia/Schleith and has completed the bores in the meantime successfully within the scheduled time frame.

The Procedure

In addition to the aforementioned difficulties, tight working conditions came into the equation. The pilot bore had to be carried out directly from the landing stage at the waterside. The bore rig had to be setup on the other side of the Rhine at the sub-station for the expanding and installation process of the four-fold pipe bundle.  

Before the bore process could start, the preliminary work for the starting pits had to be carried out. Securing the pits with sheet piles and installing casing pipes were some of the tasks. In this case approx. 30-m long steel pipes ND 600 had to be installed with an inclination of 42 degrees using the auger bore method in the installation direction, which were intended to protect the landing stage. It was important to protect the embankments against any blowouts caused by the drilling fluid under pressure. Furthermore a distance of 3 m had to be held to the landing stage.  

For the pilot bore the Grundodrill 25 N (Manufacturer: Tracto-Technik, Lennestadt) was placed in position diagonally to the landing stage. The undercarriage and the rear stabiliser just about had sufficient space between the bank edge and the starting pit and therefore it would not have been possible to use a larger bore rig. After the starting preparations the drill rods were pushed into the casing pipe.
 
After a tour through 30 m of steel pipe the bore head started to work for the approximate length of 190 m into the direction of the substation. The deepest point of the bore path was approximately 8 m below the river bed, in total at a depth of 23 m. To ensure a reliable detection a separate magnetic field had to be set up, due to the aforementioned reasons. The cables installed for this formed a square, which led from the starting point of the first bore, through the already installed pipes, then parallel to the Rhine and over the B 500 on the barrage to the starting point of the following bore.

After the first bore had been accomplished successfully, it could be used as a reference path, which could be converted with the Para-Track-Method to each of the next bores at distances of 10 metres, parallel to the bore. Intended path and actual path were clearly indicated with symbols for inclinations, depths and lengths on the laptop monitor. The implemented software checks and compares the measuring values and data continuously, which are transferred via cable by the sonde installed inside the pilot head at the front. These then show the steering orders, which are converted on the bore rig into the required thrust or rotation of the pilot rod.

With each added rod the data cable was carefully extended by a further 3 metres.

For the following expanding processes to 280-mm, 360-mm and to 580-mm bore diameter and the pulling in of the 4-fold pipe bundle, the bore rig had to be transported to the substation on the opposite Rhine embankment and placed in position, as the pre-stretch of the pipes to be installed was only possible on the landing stage. 20 m of egelen PE 100 pipes, OD 160 mm, SDR 11 in a bundle of four, each with a 220 m length, were welded and pulled in together.

Specialists had to get involved with the recipe for the drilling fluid, which had to be tuned optimally to the stabilisation of the bore canal and the reduction of friction during the pulling in process. Due to the soil structure, there were hardly any spoils and the pulling forces only reached approximately 12 tons during the installation of the pipe bundle weighing approximately 5 tons.  Inside one of the pipes, in a separate installation process, three more protection pipes 50 x 4.4 mm for control lines and the cable for the set-up of the magnetic field for the next pilot bore were installed. In the remaining three protection pipes OD 160 mm the network operator will pull in cables for the triple—phase transfer line.

For the installation itself, the pipe bundle had to be held several metres high above the dam with an excavator, to ensure the minimal permissible bending radius of the pipe was not exceeded. The installation itself took only three to four hours.

Result

Under really difficult geometrical, ground structure, detection and steering conditions the bore team of 4 completed the sophisticated bore task with great success. Several factors played a part in doing so. The fundamental aspect being the well thought through concept, which was prepared precisely. Also the very close collaboration supported by specialists for the drilling fluid, as well as special detection and steering technologies were important. As most important aspect however, we must highlight the calm and considerate co-operation of the bore team, who applied a reliable bore technology for the precise and technically efficient production of the multi-bore path.

This article was supplied by Tracto-Technik GmbH & CO KG, Lennestadt, Germany.